Funny, I don't see you stating how the middle class is defined in dollar amounts.
The middle 20% of income earners in the US is defined as those making between $32K and $55K, so it seems the Global Wage Report which is linked to in the article is correct. The median income does describe middle class.
Of course this quibbling about terms avoids the main points, that the MC has fallen behind due to compensation not commensurate with productivity and that financializaion has allowed the upper class to make even greater gains than what they have captured via wages.
The axiom that it takes money to make money is becoming even stronger.
Horatio Alger is dead and buried, meritocracy is crumbling, if you are from a wealthy family, a degree is not necessary and if you are from the lower class, a degree means little:
RIP, American Dream? Why It's So Hard for the Poor to Get Ahead Today - Matthew O'Brien - The Atlantic